If you think you would be able to land much easily on ANY motorway compared to a field you're kidding yourself. Motorways are not as straight as they seem from the air. You're very likely to hit a lamp post or centre reservation or even oncoming traffic! Even if you judged the curvature of the road perfectly, by the time you've had a chance to look ahead you'll notice a great big over-road coming up!
Sure, very James Bond like....but probably one of the worst places to land on.